Added male screw threads.

tests/d_connectors.scad

@@ -35,4 +35,5 @@ module d_connectors()
                 }
 
 if($preview)
-    d_connectors();
+    let($show_threads = true)
+        d_connectors();

tests/pillars.scad

@@ -26,4 +26,5 @@ module pillars()
         pillar(pillars[$i]);
 
 if($preview)
-    pillars();
+    let($show_threads = true)
+        pillars();

tests/rod.scad

@@ -33,4 +33,5 @@ module rods()
     }
 
 if($preview)
-    rods();
+    let($show_threads = true)
+        rods();

tests/screws.scad

@@ -34,4 +34,5 @@ for(y = [0 : len(screw_lists) -1])
     }
 
 if($preview)
-    screws();
+    let($show_threads = true)
+        screws();